Gases Brought Into Contact one with another tend to diffuse till they form a uniform mixture; and when two gases are separated by a membrane, they pass in opposite directions through it in definite proportions. The first of these laws is, in all probability, of the utmost importance in diffusing the inspired air through the residual and reserve air left in the lungs after the last expiration. But the variability which has been mentioned in the proportion of the inspired oxygen to the expired carbonic acid, affords sufficient proof that it is not by diffusion, as has sometimes been supposed, that the interchange of these gases takes place between the air and the blood. Another objection to the supposition is that the gases appear to be, at least in part, in chemical combination with the blood.

109. When respiration is obstructed, either mechanically or by deficiency or impurity of air, asphyxia or suffocation takes place. The face becomes livid with unaerated blood, the veins of the neck swollen, the circulation in the lungs stopped, and the heart gorged with dark blood, especially on the right side: there is evidence that the systemic as well as the pulmonary capillaries refuse to allow the blood to pass through them (Dalton), and death quickly supervenes.

Even a slight interference with respiration retards the circulation, and this interference may be caused by impeded expiration, as in blowing a trumpet, or violent spasmodic expiration, as in coughing, in both which cases the veins of the neck are seen to swell; or, by impeded inspiration, as in asthma, or by inhalation of a poisoned atmosphere, in which cases the hindrance to circulation is the vitiated condition of the blood.

The cessation of circulation, however, is not the cause of death by asphyxia; that is rather to be imputed to the poisonous influence of vitiated blood on the brain. Arrest of the heart's action, constituting the condition known as syncope or fainting, may be recovered from after a period of time to which it is difficult to name a limit; but asphyxia causes death in less than five minutes, and even more speedily in drowning, which is complicated by entrance of water into the lungs. The few recorded instances of recovery after submergence for longer periods are to be accounted for by supposing that the patient fainted at the moment of falling into the water, or before falling in, and so had lain without effort at inspiration.

110. It is of importance to observe that air may bo vitiated by many impurities besides carbonic acid. Principal among these are minute particles of living and dead organic matters floating in the air, and products of decomposition of organic debris. The precise nature and properties of the different substances with which the air may be filled are difficult to determine. It must not be supposed that things which are offensive to the senses are necessarily deleterious to the health. There is no proved relationship between the intensity of bad smells and insalubrity of the air, and many invectives about poisoned atmospheres, sufficiently excellent in their general tendency, are based on very slender scientific foundation. But while it is admitted that unpleasant odours are not always injurious, there can be no doubt that constant exposure to the emanations of putrefaction, especially such as is fed with meagre supplies of oxygen, engendering products of unstable equilibrium, is thoroughly baneful, and may exert its noxious influence with but little warning given to the nostrils.

All disinfectants in use act in one or other of two ways: they either decompose organic matter, or they preserve or pickle it; permanganate of potash, chlorine, and fumes of burning sulphur being examples of the destructive kind, while creosote and carbolic acid are instances of the preservative or antiseptic description.